1. Field of the Invention
The present invention relates to an eddy current testing method of a metal material such as a steel pipe or tube having magnetism, a steel pipe or tube tested by the eddy current testing method, and an eddy current testing apparatus for carrying out the eddy current testing method. In particular, the present invention relates to an eddy current testing method capable of securely detecting a high hardness part locally occurring in a metal material, and capable of securely checking whether the high hardness part has been removed after execution of a repairing treatment such as grinding for removing the high hardness part, a steel pipe or tube tested by the eddy current testing method, and an eddy current testing apparatus for carrying out the eddy current testing method. Hereinafter, “pipe or tube” is referred to as “pipe” when deemed appropriate.
2. Description of the Related Art
In a production process of a metal material such as a steel pipe, microstructure of the metal material locally changes due to embrittlement of the metal material resulting from microstructure changes such as carburization, decarbonization, deposition of embrittled phase during heat treatment, due to collisions between metal materials or collisions between the metal material and transporting equipment during transportation, or due to a strong working caused by seizure during a cold working. It is known that a local high hardness part arises, and the high hardness part occasionally has a hardness higher than that of the remaining unchanged part, by 50 Hv or larger by Vickers hardness. When such a local high hardness part occurs in a metal material, there arises a fear of breakage due to embrittlement of the metal material and deterioration in corrosion resistance in the high hardness part.
For this reason, it is necessary to detect a local high hardness part occurring in a metal material, and to check whether the high hardness part has been actually removed after execution of a repairing treatment (treatment such as grinding) for removing the high hardness part.
However, detecting a high hardness part and checking removal of the high hardness part by visual inspection, or by using a simplified hardness meter which measures hardness by magnitude of press fit or ultrasonic resonance frequency of an indenter, after press-fitting the indenter into a metal material inconveniently spend time due to difficulty in continuous measurement, and cause variation in determination. Accordingly, if such a local high hardness part can be detected in a non-contact or nondestructive manner, and whether the high hardness part has been actually removed can be checked in a non-contact or nondestructive manner after execution of a repairing treatment for removal of the high hardness part, the efficiency and reliability of detection of high hardness part and checking of removal of high hardness part will be improved.
As a technique of detecting hardness of a metal material or a part where hardness changes in a non-contact or nondestructive manner, for example, Japanese Unexamined Patent Publication No. 58-102148 discloses the art utilizing the fact that the magnetic field having changed due to magnetization of a steel plate (transmitted magnetism) correlates with hardness of the steel plate. Japanese Unexamined Patent Publication No. 59-108970 discloses the art utilizing the fact that magnetic characteristics (coercive force, residual magnetization, saturation magnetization, magnetic permeability, hysteresis loss) of a steel material have correlation with mechanical properties (hardness, burning depth, strength, crystal grain size) of the steel material. Japanese Unexamined Patent Publication No. 60-185158 discloses the art of detecting change in material quality (hardness, carbon content) or nature of a steel pipe using a bridge circuit including a test coil and a comparison coil. Japanese Published Patent Publication No. 9-507570 discloses the art of estimating hardness of steel by measuring a plurality of magnetic parameters of steel.
Japanese Unexamined Patent Publication No. 8-178902 discloses the art of detecting an anomalous microstructure defective part in which a part of surface of a steel plate is carburized and crystal structure is refined, using an eddy current sensor of magnetic saturation type. Japanese Unexamined Patent Publication No. 62-147356 discloses a sigma phase testing method of stainless steel member using an eddy current testing apparatus.
Also Japanese Unexamined Patent Publication No. 2003-232777 discloses a method of detecting an unturned part remaining after a turning process for removing a surface decarbonization layer of steel pipe or round-bar steel occurring due to a heat treatment, by utilizing eddy current.
As described above, it is known that by measuring change in magnetic characteristics of a metal material, for example, by an eddy current testing method, a part where mechanical property such as hardness of the metal material changes can be detected in a nondestructive manner, and the method of checking whether an anomalous part has been removed after a repairing treatment by an eddy current testing method is also known. Therefore, one can conceive of detecting a high hardness part locally occurring in a metal material by using an eddy current testing method, and checking whether the high hardness part has been actually removed by the eddy current testing method after execution of a repairing treatment for removing the high hardness part, by application of the above known arts.
However, when the metal material is a magnetic material, it is sometimes the case that accurate detection of a high hardness part is difficult because a detection signal resulting from magnetism variation (magnetism unevenness) which is inherent to the metal material, and a detection signal resulting from distance (liftoff) variation between the metal material and a detecting coil are superimposed as noises, with respect to a detection signal in the local high hardness part at the time of carrying out an eddy current testing. Further, the region where the high hardness part is removed by grinding or the like shows larger noise signal due to liftoff variation because the surface of the metal material is ground. Therefore, there is a demand for an eddy current testing method capable of detecting a local high hardness part more reliably.